"SiAlON"s are phases in silicon-aluminum-oxygen-nitrogen and related systems, comparable in variety and diversity with mineral aluminosilicates. They contain one-, two-, and three-dimensional arrangements of silicon oxide tetrahedra in which silicon and oxygen atoms are partially replaced by aluminum and nitrogen. See K. H. Jack, "Sialons and related nitrogen ceramics," 11 J. of Materials Sci. 1135-58 (1976). Ceramic materials made from SiAlON typically have high toughness, and elevated temperature strength and oxidation resistance. These properties have made SiAlON ceramics a desirable candidate for many high temperature industrial applications.
In an attempt to provide a ceramic SiAlON composition which is usable in high temperature applications, prior art methods and compositions have taught the combination of alpha-SiAlON with beta-SiAlON. Typically, alpha-SiAlON, which appears mostly as fine equiaxed grains in the microstructure of the material, is associated with hardness in the material. On the other hand, beta-SiAlON mostly appears as elongated fiber-like grains in the microstructure. See Hwang et al., U.S. Pat. No. 5,227,346. Since, the beta-SiAlON material is elongated, it adds strength and fracture toughness to the material. Consequently, it is an advantage for a material made of SiAlON to incorporate both alpha-phase and beta-phase SiAlON. By varying starting materials in the SiAlON composition, it is possible to vary the alpha- to beta-SiAlON phase ratio. This will give rise to a series of materials where hardness and fracture toughness can be tailored.
A common problem with multi-phase SiAlON sintered bodies is that one or more minor phases, generally intergranular amorphous (glassy) phases, are formed at grain boundaries between the alpha- and beta-SiAlON phases. These intergranular glasses are undesirable because they generally cause high temperature degradation and reduction of the overall strength of the ceramic material. The intergranular glasses also cause the bodies to have lower oxidation resistance, especially at high temperatures. This may lead to reduced mechanical reliability such as load-bearing capability of the sintered bodies. For a discussion of negative implications of oxidation in ceramics, see Tressler, "High-Temperature Stability of Non-Oxide Structural Ceramics", 18[9] MRS Bull. 58-63 (1993). For this reason, it would be advantageous to provide a multi-phase SiAlON material with minimal or no intergranular glasses.
Prior art compositions and methods have attempted to rid the SiAlON materials of these glassy phases. For example, since oxide sintering additives contribute to the formation of glasses which remain at grain boundaries, attempts have been made to eliminate these additives from starting materials. However, these methods typically produce ceramic bodies that are difficult, if not impossible, to fully densify. Eliminating the additives from the starting materials also changes the microstructure of the resulting sintered bodies, inhibiting formation of elongated grains, and thus, impairing mechanical properties.
Another method of removing the glasses from a ceramic body is a post-fire heat treatment. In this method, a densified ceramic body containing an intergranular amorphous phase is exposed to temperatures between about 1000.degree. C. and 1600.degree. C. in order to promote crystallization of the glassy phase. The crystallized phase provides better resistance to degradation at higher temperatures than the glass. However, a problem with post-fire heat treatment is that complete crystallization may be inhibited by kinetic factors such as a large volume change upon crystallization that causes stress in glass residues which are constrained by surrounding grains. Even if complete crystallization occurs, the temperature at which strength degradation is observed is only raised to a temperature somewhere between the glass melting point (typically about 900.degree. C. to 1000.degree. C.) to that of an eutectic temperature along the SiAlON grain-boundaries (typically about 1200.degree. C. to 1500.degree. C.).
Another method of removing glass from ceramic bodies is by chemical or thermal leaching. Such a process is described in Clarke, "Thermodynamic Mechanism for Cation Diffusion Through an Intergranular Phase: Application to Environmental Reactions with Nitrogen Ceramics," Progress in Nitrogen Ceramics 421 (Martinus Nijhoff Publishers 1983). However, this type of method is complicated, too tedious, not efficient, and is generally difficult to control.
In order to encourage reduction of the volume percent of intergranular amorphous phase, some methods add higher amounts of AlN to the SiAlON composition. See, e.g.'s, T. Ekstrom and M. Nygren, "SiAlON Ceramics", 75[2] J. Am. Ceram. Soc. 259, 268 (1992) (addition of an "excess" 2 wt % AlN to the starting mixture), and T. Ekstrom, "Preparation and Properties of Alpha--Si--Al--O--N Ceramics," 3[2] J. of Hard Materials 109, 113-14 (1992). However, these methods have problems similar to those described above and, typically, only reduce the glass to a range of between about 3 to 6 volume percent. The methods are further complicated in multi-phase SiAlON systems. In multi-phase systems, due to intricate phase relationships, even slight changes in starting compositions produce numerous undesirable end products. For example, see Sun et al., "Subsolidus Phase Relationships in Part of the System Si, Al, Y/N, O: The System Si.sub.3 N.sub.4 --AlN--YN--Al.sub.2 O.sub.3 --Y.sub.2 O.sub.3," 74[ 11] J. Am. Ceram. Soc. 2753-58 (1991). Phase relationships are further affected by surface oxides, inherent in nitride raw materials. In most cases, the surface oxides result in either intergranular amorphous phases or undesirable crystalline phases. Thus, adding higher amounts of AlN in order to significantly lower the glass content (e.g.&lt;3 volume percent glass) has not been applied to SiAlON ceramics having both alpha- and beta-SiAlON phases.
It would be desirable to have a method of forming a dense multi-phase SiAlON ceramic material having no greater than three volume percent of intergranular amorphous phase, but not having the above identified problems.